Summary Interleukin-4 is a prototypic type I cytokine that is the central regulator of allergic inflammatory responses. It controls the polarization of naive CD4 T cells to the TH2 phenotype and Ig class switching to IgE. The Cytokine Biology Unit has characterized the signaling mechanisms utilized by the IL-4 receptor. It has shown that activation of the latent transcription factor, Stat 6, controls both TH2 polarization and IgE class switching. Scientists in the Unit have established that both GATA3 and Stat5 are essential for acquisition of IL-4 producing capacity. They have recently carried out genome wide analyses of histone modification and transcription factor binding in each of the various Th lineages to gain insight into the processes underlying Th differentiation. They have made good progress in the study of the sites in the genome to which the master regulatory transcription factor GATA3 binds. This information coupled with knowledge of the binding sites for STAT5 promises to give important insights into the genetic regulation of Th2 differentiation. Comprable studies on the other Th lineages should provide similar information for the differentiation of those cells. Unit scientists have now been shown that T cell receptor-mediated activation of the erk signaling pathway blocks Th2 differentiation by preventing transcription of GATA3 and desensitizing the IL-2 receptor. Strong TCR-mediated signals stimulate erk phosphorylation, thereby preventing TH2 differentiation and accounting for poor Th2 differentiation at high antigen concentration. Low concentrations of antigen, which activate erk only weakly, are permissive for early IL-4 production and TH2 differentiation. Unit scientists have also shown that Notch signaling participates in Th2 primiing but that it doers so not by regulating GATA3, which others had proposed, but rather by regulating IL-2 production. Limited IL-2, by limiting STAT5, diminishes Th2 priming. The defect in Th2 priming in mice with defects in the Notch pathway can be rescued by the addiiton of IL-2. Gfi-1, in its role as a transcriptional repressor, is critical for robust growth to IL-2, since it appears to repress expression of genes such as SOCS1 and SOCS3 that inhibit IL-2 mediated growth. On the other hand, Gfi-1 diminishes growth in response to IL-7 since it mediates TCR- and cytokine-mediated down regulation of the IL-7R alpha chain. These studies, utilizing Gfi-1 conditional knockout mice developed in the Unit, establish that Gfi-1 plays a critical role in lymphocyte homeostasis and that precise regulation of Gfi-1 expression is central to a balanced population of CD4 and CD8 T cells. Further, Gfi-1 plays a central role in the phenotype of memory CD4 and CD8 cells induced by viral infection. More recently, it has been shown that Gfi-1 represses differentiation to the IL-17 and T%reg fates. Indeed, Gfi-1 appears to function as a repressor of fates alternate to Th2. Recently, Unit scientists have begun the analysis of the role of IL-1 family members in the regulation of T cell function. They have shown that IL-1 is a potent enhancer, acting directly on responding CD4 T cells, of cellular expansion in response to antigen. Addition of IL-1 to an immunization protocol can enhance T cell expansion by as much as 10-fold and strikingly enhance priming to Th17 ce;lls and to a lesser extent to Th2 cells. Uni scientists have also shown that differentiated Th effector cells can be stimulated to secrete key cytokines by stimulation with an IL-1 family member and by STAT activation. For Th1 cells, the IL-1 family member required is IL-18 and the STAT is STAT4;for Th2, IL-33 and STAT5 and for Th17, IL-1 and STAT3. In addition, an in depth effort to understand the distinctive signaling mechanisms used for IL-4 and IL-13 responses, with emphasis on the differential roles of the type I and type II IL-4 receptors and how different cell types show very different relative sensitivity to these congeners. For this purpose, responses of various macrophage populations from wild-type and gamma common-deficient mice to IL-4 and IL-13 have been analyzed in detail. Both the phosphorylation of STAT6, as measured by glow cytometric analysis, and the induction of arginase-1, a gene that is rapidly induced through the IL-4 receptor, has been evaluated. Bone marrow derived macrophage (BMDM) populations and monocytes are exquisitely sensitive to IL-4 but require 100 to 1000 fold more IL-13 to achieve a comparable response. Paradoxically, BMDM from gamma common-deficient mice respond well to IL-13 but poorly to IL-4 and anti- gamma common antibody essentially abolishes responses of monocytes to IL-4. Peritoneal macrophages and fibroblasts respond well to both IL-4 and IL-13 and gamma common deletion has very little effect on either IL-4 or IL-13 responses. Using information regarding the numbers of receptor subunits and equilibrium constants for each interaction, a model has been derived to explain these results. From the analysis, it has been concluded that IL-4 plays a central role as an inducer of differentiation while IL-13 is a major effector molecule. LI scientists have also developed new insights into the production of IL-4 by basophils. They have shown that these cells are massively recruited into the tissues, spleen and blood in helminth infection and that this recruitment is T cell dependent. IL-3 appears to play a major role in basophil recruitment. Laboratory scientists have developed a series of indicator mice. Particularly valuable are those that reflect the expression of IL-4 and IL-13. These indicator mice were made through the introduction of a bacterial artificial chromosome in which the AM-Cyan gene replaced IL-4 and destabilized DS-Red has replaced IL-13. These mice have proved to be excellent reporters of cytokine production in Schistomiasis infection and thier value in other infections is being studied.